Long-Term Evolution (“LTE”) is an effort to develop advanced wireless mobile radio technology that aims to succeed current Third Generation (“3G”) telecommunication standards and technology for mobile networking. The actual standard is known as the International Telecommunication Union (“ITU”) 3rd Generation Partnership Project (“3GPP”), Release 8, although the term LTE is often used to reference the standard. LTE is considered by many to be a Fourth Generation (“4G”) technology, both because it is faster than 3G, and because, like the Internet, LTE uses a flat “all-IP” architecture where all information, including voice, is handled as data.
The LTE standard presently supports two modes of data allocation: localized and distributed. Localized transmission is intended for frequency selective scheduling (FSS), while distributed transmission is intended to maximize the amount of frequency diversity when sub-band channel knowledge is not available or out-of-date at the scheduler. The network entity usually in charge of resource scheduling is the base station defined in 3GPP LTE systems as an enhanced Node B (eNode B) for radio communication systems.
In order to improve the performance of the LTE systems, several improvements have been introduced in the standards, like the use of Orthogonal Frequency Division Multiplexing (OFDM) based techniques for the radio interface. OFDM is based on the fact that the different (orthogonal) sub-carriers may be used in parallel to transmit data over the air interface. The controllable radio resource in OFDM networks has three aspects: frequency, time and space. A Physical Resource Block (“PRB”) is a set of time frequency resources whose size is the minimum resource allocation size. Each so-called PRB is defined by its frequency extension (180 kHz) and its time extension (0.5 ms), and data are transmitted over one or more PRBs consisting of a set of contiguous sub-carriers and having a predefined time extension. In the LTE standard, the generic frame structure is defined by 10 ms (10 milliseconds) frames, divided into ten 1 ms subframes, that are composed by two 0.5 ms slots. Resources are allocated in a per subframe basis. The minimum amount of resources allocated to an UE is two PRBs, each one transmitted over each slot of the subframe (the same amount of PRBs is assigned to a UE in the two slots of a subframe).
One of the advantages of using OFDM in the LTE radio interface is the possibility of supporting frequency selective scheduling based on the Channel Quality Index (CQI) reports provided by the User Equipment (UE) and the estimations performed by the eNodeB (based on the sounding reference signals sent by the UE to assist the network in allocation of appropriate frequency resources for uplink transmission). This feature takes advantage of the multipath propagation conditions that are common in mobile communications.
The performance of the LTE radio access technology is affected in environments where the LTE system is dimensioned (i.e., the number of base stations to be installed) attending to capacity requirements rather than coverage's ones, e.g., deployments in dense urban areas (with a high density of eNodeBs). In these environments the distance between macrocellular base stations (eNodeBs) is relatively small (as low as 150-200 meters), which results in propagation conditions with a higher proportion of Line of Sight (LoS) propagation (i.e., where there area no obstacles between the transmitting and receiving antennas that obstruct the radio link). It is considered that the sophisticated FSS mechanisms proposed for LTE are less effective in this kind of environments due to the fact that the coherence bandwidth of the propagation channels is relatively large with respect to the system bandwidth, reducing the opportunistic gain associated with this kind of algorithms. There are studies indicating that in a number of deployment scenarios where capacity, rather than coverage, may be the limiting factor, propagation conditions are such that coherence bandwidth is relatively large compared to the ones' considered in the usual propagation models employed for the evaluation of the standards by 3GPP, ITU-R, IEEE and other standardization bodies.
The coherence bandwidth is a statistical measurement of the range of frequencies over which the channel can be considered “flat”, i.e., some of the signal's spectral components falling outside the coherence bandwidth will be affected differently (independently), compared with those components contained within the coherence bandwidth. Most of the evaluations of LTE scheduling algorithms are based on the use of standardized channel models that have a coherence bandwidth smaller than 1.5 MHz. However, estimations carried out in realistic simulation environments (using 3D cartography and real sites' locations) show that in some areas (LoS, border of the cell, . . . ) coherence bandwidth significantly exceeds this value.
In order to overcome these problems, there is the possibility of using transmit diversity technologies like Cyclic Delay Delivery (CDD), which provides transmit diversity without requiring the modification of the receiver and is supported in other OFDM systems like the digital television standard DVB-T. However, in the Release 8 of the LTE standard, this feature is only supported associated with the support of open loop spatial multiplexing and not as a pure transmission diversity procedure. But in some of the situations (e.g., cell borders), the use of MIMO spatial multiplexing techniques is not feasible.